A high-frequency amplifying device that uses a saturated amplifier based on linear amplification with nonlinear components (LINC) is known as a highly-efficient linear amplifying device. FIG. 9 is a block diagram of a conventional amplifying device. As depicted in FIG. 9, the amplifying device includes a signal separating unit, digital to analog converters (D/As) 2 and 12, low pass filters (LPFs) 3 and 13, quadrature modulators (QMODs) 4 and 14, amplifiers 5 and 15, a local oscillation unit 6, and a combining unit 7.
The signal separating unit is connected to an input terminal 8, and separates an input signal (Sin(t)) input from the input terminal 8 and having a variable envelope into a signal pair (Sc1(t), Sc2(t)) having a phase difference according to the amplitude of the input signal (Sin(t)). For example, the input signal (Sin(t)) is an amplitude-modulated signal or a phase-modulated (angle-modulated) signal. Each signal of the signal pair (Sc1(t), Sc2(t)) is a phase-modulated signal having a constant envelope and a constant amplitude. The input signal (Sin(t)) and the signal pair (Sc1(t), Sc2(t)) may be a baseband signal or an intermediate frequency (IF) signal. The signal pair (Sc1(t), Sc2(t)) is generated as a digital signal by the signal separating unit 1.
D/As 2 and 12 are connected to the signal separating unit 1. The D/A 2 converts one signal (Sc1(t)) of the signal pair (Sc1(t), Sc2(t)) output from the signal separating unit 1 into an analog signal. The D/A 12 converts the other signal (Sc2(t)) of the signal pair (Sc1(t), Sc2(t)) output from the signal separating unit 1 into an analog signal.
The LPF 3 is connected to the D/A 2, extracts a component corresponding to the band of frequency of the signal Sc1(t) from the signal output from the D/A 2, and suppresses frequency components other than the band of frequency of the signal Sc1(t). The LPF 13 is connected to the D/A 12, extracts a component corresponding to the band of frequency of the signal Sc2(t) from the signal output from the D/A 12, and suppresses frequency components other than the band of frequency of the signal Sc2(t).
The QMOD 4 is connected to the LPF 3. The QMOD 4 performs a quadrature modulation on the signal output from the LPF 3 using a high-frequency oscillation signal (SL(t)) output from the local oscillation unit 6, and generates one signal (S1(t)) of the high-frequency signal pair (S1(t), S2(t)) that is a radio frequency (RF) signal. The QMOD 14 is connected to the LPF 13. Similar to the QMOD 4, the QMOD 14 performs a quadrature modulation on the signal output from the LPF 13, and generates the other signal (S2(t)) of the high-frequency signal pair (S1(t), S2(t)).
For example, it is assumed that the signal (Sin(t)) input to the signal separating unit 1 is represented by the following equation (1). In this case, the signal pair (Sc1(t), Sc2(t)) output from the signal separating unit 1 and the signal pair (S1(t), S2(t)) output from the QMOD 4 are represented by the following equations (2) to (6).Sin(t)=a(t)·cos [θ(t)]  (1)Sc1(t)=amax·cos [θ(t)+ψ(t)]  (2)Sc2(t)=amax·cos [θ(t)−ψ(t)]  (3)S1(t)=amax·cos [2·π·fc·t+θ(t)+ψ(t)]  (4)S2(t)=amax·cos [2·π·fc·t+θ(t)−ψ(t)]  (5)ψ(t)=cos−1[a(t)/(2·amax)]  (6)
In the equations (1) to (6), a(t) is the amplitude modulation of Sin(t); θ(t) is the phase modulation (angle modulation) of Sin(t); fc is the frequency of SL(t) and is the carrier frequency of S1(t) and S2(t); and amax is a constant that is set according to the saturation output level of a pair of amplifiers 5 and 15 described later. Thus, the signal separating unit 1, the local oscillation unit 6, and the QMODs 4 and 14 enable a generation of a high-frequency signal pair S1(t) and S2(t) that is phase-modulated such that the phase difference 2×ψ(t) is generated according to the amplitude of Sin(t).
The amplifying unit 5 is connected to the QMOD 4, and amplifies the signal (S1(t)) output from the QMOD 4. The amplifying unit 15 is connected to the QMOD 14, and amplifies the signal (S2(t)) output from the QMOD 14. The amplifying units 5 and 15 are provided in parallel, and have substantially the same gain and the same phase characteristics. The signal output from the amplifying unit 5 is GxS1(t) and the signal output from the amplifying unit 15 is GxS2(t), where G is the gain of the amplifiers 5 and 15. The amplifiers 5 and 15 are used as saturated amplifiers.
The combining unit 7 is connected to the amplifiers 5 and 15, and combines the signal (G×S1(t)) output from the amplifying unit 5 and the signal (G×S2(t)) output from the amplifying unit 15. The combining unit 7 is connected to an output terminal 9, and outputs the combined high-frequency signal (Sout(t)) from the output terminal 9. For example, the combined high-frequency signal (Sout(t)) is represented by the following equation (7), where φ is the transmission phase of the signal (S1(t)) output from the QMOD 4 and the signal (S2(t)) output from the QMOD 14.
                                                                        Sout                ⁡                                  (                  t                  )                                            =                            ⁢                                                G                  ·                                      a                    max                                    ·                                      cos                    ⁡                                          [                                                                        2                          ·                          π                          ·                          fc                          ·                          t                                                +                                                  θ                          ⁡                                                      (                            t                            )                                                                          +                                                  ψ                          ⁡                                                      (                            t                            )                                                                          +                        ϕ                                            ]                                                                      +                                                                                                      ⁢                              G                ·                                  a                  max                                ·                                  cos                  ⁡                                      [                                                                  2                        ·                        π                        ·                        fc                        ·                        t                                            +                                              θ                        ⁡                                                  (                          t                          )                                                                    -                                              ψ                        ⁡                                                  (                          t                          )                                                                    +                      ϕ                                        ]                                                                                                                          =                            ⁢                              2                ·                G                ·                                  a                  max                                ·                                  cos                  ⁡                                      [                                                                  2                        ·                        π                        ·                        fc                        ·                        t                                            +                                              θ                        ⁡                                                  (                          t                          )                                                                    +                      ϕ                                        ]                                                  ·                                                                                                      ⁢                              cos                ⁡                                  [                                      ψ                    ⁡                                          (                      t                      )                                                        ]                                                                                                        =                            ⁢                              G                ·                                  a                  ⁡                                      (                    t                    )                                                  ·                                  cos                  ⁡                                      [                                                                  2                        ·                        π                        ·                        fc                        ·                        t                                            +                                              θ                        ⁡                                                  (                          t                          )                                                                    +                      ϕ                                        ]                                                                                                          (        7        )            
As represented by the equation (7), the amplifying device depicted in FIG. 9 can obtain a high-frequency output signal Sout(t) that is the signal Sin(t) input to the signal separating unit 1 and amplified by the gain G, and can perform highly-efficient linear amplification on the input signal.
As such an amplifying device using a saturated LINC amplifier, an amplifying device is known that separates an input signal into the in-phase signals and the quadrature signals, amplifies the signals respectively according to the method described above, and orthogonally adds the signals again. An amplifying device is also known that feeds back the signal output from the amplifying device, and corrects the phase of each signal of the signal pair separated by a signal separating unit. Related technologies are described in, for example, Japanese Laid-open Patent Publication Nos. 2007-150905, 2004-260707, and 2007-174148.
However, conventional amplifying devices have the following problem. When a signal of which carrier polarity reverses, such as the phase shift keying (PSK) signal, is input to the conventional amplifying devices, the digital signal pair generated by the signal separating unit includes a point where the phase reverses by 180 degrees, thereby increasing the band width significantly. However, a digital signal can represent only up to half of the sampling frequency due to the Nyquist theorem. As a result, the signal that is converted into an analog signal by the D/A and from which folded components are removed by the LPF includes high ringing, and becomes a different signal from a constant-envelope signal.
FIGS. 10 and 11 are diagrams of an example of the constellation of the output signal pair when the signal input to the conventional amplifying devices has 2 tones. FIG. 10 illustrates the constellation of the output signal pair before the analog conversion. FIG. 11 illustrates the constellation of the output signal pair that has passed through the LPF (smoothing filter) after the analog conversion. The output signal pair after passing through the smoothing filter includes an amplitude component due to the influence of ringing. As a result, the combined high-frequency output signal is deteriorated and distorted if the output signal pair after the analog conversion is affected by AM/AM distortion (amplitude distortion) and/or AM/PM distortion (phase distortion) during the amplification.